A fiber-reinforced composite material has been widely applied as a structural material for an aircraft, an automobile, civil engineering and construction, sporting goods, and the like because the material includes reinforcing fibers, such as glass fibers, aramid fibers, or carbon fibers, and a thermosetting matrix resin, such as an unsaturated polyester resin, a vinyl ester resin, an epoxy resin, a phenol resin, a benzoxazine resin, a cyanate resin, or a bismaleimide resin, has a light weight, and is excellent in mechanical properties, such as a strength, corrosion resistance, and fatigue resistance.
The production of the fiber-reinforced composite material is performed by an approach, such as an autoclave molding method or a press molding method involving using a prepreg obtained by impregnating the reinforcing fibers with the thermosetting matrix resin in advance, or a wet lay-up molding method, a pultrusion molding method, a filament winding molding method, or a RTM method including a step of impregnating the reinforcing fibers with a liquid matrix resin and a molding step based on heat curing. In the wet lay-up molding method, the pultrusion molding method, the filament winding molding method, or the RTM method out of those methods, a matrix resin having a low viscosity is used in order that the reinforcing fibers may be immediately impregnated with the resin.
In addition, in the wet lay-up molding method, the pultrusion molding method, or the filament winding molding method out of those methods, a matrix resin showing a small viscosity increase ratio during the step of impregnating the reinforcing fibers with the resin is used for securing stable impregnability.
With regard to the tensile elongation at break of each of the reinforcing fibers to be used in the fiber-reinforced composite material, in general, the glass fibers each show a value of from 3% to 6%, the aramid fibers each show a value of from 2% to 5%, and the carbon fibers each show a value of from 1.5% to 2.0%. Accordingly, a material having a tensile elongation at break higher than that of any such reinforcing fiber is desirably applied as the matrix resin for obtaining a fiber-reinforced composite material excellent in strength.
In the wet lay-up molding method, the pultrusion molding method, or the filament winding molding method, a thermosetting resin, such as an unsaturated polyester resin, a vinyl ester resin, or an epoxy resin, has heretofore been used. Each of the unsaturated polyester resin and the vinyl ester resin each having radical polymerizability has a low viscosity and is excellent in fast curability, but involves a problem in that the mechanical properties of a molded product, such as heat resistance, a strength, and toughness, are relatively low. Meanwhile, the epoxy resin provides a molded product having high heat resistance, a high strength, and high toughness, but involves a problem in that the viscosity of the resin is relatively high.
A bisphenol A-type epoxy resin is used as the matrix resin of the fiber-reinforced composite material because the resin is excellent in economical efficiency and physical properties. However, a bisphenol F-type epoxy resin having a low viscosity is also used for improving impregnability into the fibers. However, the bisphenol F-type epoxy resin contains an epoxy group-containing component that is trifunctional or more owing to its production method, and hence provides a resin composition showing a fast gelation rate, that is, a large viscosity increase ratio at the time of the mixing of a curing agent and a curing accelerator. Accordingly, stable impregnability into the fibers is impaired in a process in which a long-term impregnation step is required.
In Patent Literature 1, there is a proposal of a low-viscosity resin composition for a fiber-reinforced composite material using a bisphenol F-type epoxy resin. In addition, in Patent Literature 2, there is a proposal of a low-viscosity resin composition for a fiber-reinforced composite material obtained by using a high-purity bisphenol F-type epoxy resin. However, in each of those literatures, no reference is made to each component in the bisphenol F-type epoxy resin, and no reference is made to the viscosity increase ratio of the resin composition.
In Patent Literature 3, an attempt is made to achieve both a reduction in viscosity increase ratio and fast curability in a resin composition formed of an epoxy resin and an acid anhydride-based curing agent, the resin composition serving as a matrix resin to be used in the pultrusion molding method, through the use of a specific curing accelerator. However, in order that the viscosity increase ratio may be further reduced and hence impregnability that is stable over a long time period may be secured, attention needs to be paid to the amount of each component in the bisphenol F-type epoxy resin.
In each of Patent Literatures 4 and 5, there is a description of a reduction in viscosity of a resin composition through the use of an alicyclic epoxy resin or a reactive diluent, such as an aliphatic glycidyl ether. The alicyclic epoxy resin has a low viscosity and provides a molded product having high heat resistance after its curing, but involves a problem in that the toughness of the molded product is low owing to the high crosslink density of the resin. The reactive diluent, such as the aliphatic glycidyl ether, has a low viscosity but involves a problem in that the heat resistance of a cured product of the resin composition reduces.
In each of Patent Literatures 6 and 7, a reduction in viscosity is achieved by blending a resin composition formed of an epoxy resin and an acid anhydride or an amine-based curing agent with a radical-polymerizable compound, such as acrylic acid. However, in each of those literatures, a radical-polymerizable compound containing an acid group is used, and hence a reaction between an epoxy group and a carboxyl group advances at the time of the mixing of all components. Accordingly, an increase in viscosity based on an increase in molecular weight occurs, and hence a problem in terms of the stability of a long-term impregnation step occurs.
With regard to the matrix resin of the fiber-reinforced composite material, an attempt has been made to achieve both an improvement in impregnability by a reduction in viscosity of a resin composition and the impartment of heat resistance to a molded product. However, in addition to the foregoing, an improvement in toughness of the molded product, in particular, an improvement in tensile elongation amount thereof has been further desired.